International Journal of Engineering and Technical Research (IJETR)

ISSN: 2321-0869, Volume-1, Issue-9, November 2013

Interaction of Flow and Sediment Discharge

To Formation of Point Bars That Lead to Bifurcation – A Review D.A. Tholibon and J. Ariffin Bifurcation occurs when a river splits into two branches or ABSTRACT - This paper highlights the progress in theory and a middle bar forms in a channel and commonly occurs inexperimentation of previous researches on the interaction of anabranch reach [4].flow and sediment discharge to formation of point bars that Local water and sediment transport division need to belead to bifurcation. Numerous studies had made emphasis to the understood because it play important role in order tomechanics of point bar development that include chute cutoff determine the avulsing and river bifurcation and this isand transverse bar conversion. These point bars formation arecontrolled by the threshold of width to depth ratio of the emphasized by [5]. A good understanding of sedimentchannel, depending on the shields stress and roughness. The distribution process at river bifurcation is crucial for rivercomplexity of flow and sediment distribution at bifurcation management [6] as this determines which downstreamwould depend on many factors namely the nose angle, branch will have the higher discharge. This is important incross-sectional areas and slopes of the downstream channels. In order to avoid the flooding risk and navigability of bothaddition, the stability of the bifurcation depends on the shield branches as well as the availability of water for vegetation,stress and incoming flow of the river. In summary, the division agricultural and human needs.of flow and sediment at bifurcation are affected by regional as [3] concluded that much of the theoretical analysis haswell as local factors. been on the formation of bars and development from straight Index Terms— bifurcation, braiding, flow and sediment to meandering or transition from meandering to braiding butdischarge, point bars. very little has considered the dynamics and variability of bars within a meandering natural reach. Nevertheless, the flow I. INTRODUCTION patterns that cause by point bars and bifurcation need further investigation. This paper is devoted to providing a review on Rivers offer essential habitats and serve as feeding and the achievement by previous investigators on the aspect ofbreeding grounds for a wide range of reverie biodiversity that flow and sediment interaction that lead to formation oflives in the river as well as in the river-fringing vegetation. midland point bars and bifurcation.However, the erosion of river bank and the deposition ofsediment along the river may cause the river to meander andalter the original form of the river fluvial. The meandering II. PHYSICAL AND LABORATORY STUDY OF RIVER BIFURCATIONriver may generate river bifurcation that may influence thestability of the river system. Bifurcations have been identified as the primary cause of As suggested by field observations and flume experiments braiding [7]. Subsequently, laboratory models have for manyby [1] the bifurcation is typically the consequence of central years been the preferred tool to investigate their dynamics.bar deposits. According to [2] central bar is deem as the Explanations of the causes of braiding in alluvial streams fall`molecule' of braided rivers. A braided river comprises of a into two general categories. The first is a functionalnetwork of interlaced channels which display a significant explanation relating to the occurrence of braiding to avariation in width, both of the individual channel segments particular combination of environmental factors such asand of the whole channel ensemble. These width variations stream discharge, channel or valley gradient, sedimentseem to be associated with bank erosion, bar development particle size and bank resistance. Secondly through stabilityand channel curvature. analysis of channel, scale bed forms in two phase flow. These Study by [3] found out that point bars are the dominant two approaches consider the physical sedimentary processestype of bar. accompanying the onset of braiding that lead to formation of bifurcation. [8] measured the braiding mechanism using the flume of 10m long and 2m wide with varied combinations of constant discharge and slope but identical bed material particle size distributions. It concluded that braiding in any river depend Manuscript received November 13, 2013. on local flow and sediment transport conditions at points D.A. Tholibon, Fluvial & River Engineering Dynamics (FRiEnD), where braiding occurs and also the sediment mobilityInstitute of Infrastructure Engineering & Sustainable Management (IIESM), conditions in the stream and general flow. It also concludedFaculty of Civil Engineering, Universiti Technologi MARA (UiTM), 40450 that the most frequent braiding mechanism is chute cut offShah Alam, Selangor, Malaysia.:+6-017-604-9749. J. Ariffin, Fluvial & River Engineering Dynamics (FRiEnD), Institute of and transverse bar conversion.Infrastructure Engineering & Sustainable Management (IIESM), Faculty of [9] performed series of experiments on stable bifurcationCivil Engineering, Universiti Technologi MARA (UiTM), 40450 Shah that form from central point bars deposits. Bifurcation isAlam, Selangor, Malaysia.;+6-017-300-7246 being unstable if single thread being supplied with unequal

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flow and sediment. [9] had also conducted on flume roughness, channel aspect ratio, there is only one asymmetricconsisting the straight constant width channel followed by discharge ratio (Qr) for which the downstream bifurcatelinearly diverging reach, ending in a much wider constant channels are stable compare to small perturbations.width channel. It is observed that large shield values of [17] provides detailed field analysis study of bifurcationincoming parameter, the bifurcation was stable and within anabranching cobble gravel rivers. It showed thatsymmetrical. [10] studied the physical model of gravel bifurcation in anabranching rivers are dynamics, complexbraided rivers to investigate the adjustment of braiding and both local channel conditions and longevity of fluvialintensity to step changes in channel forming discharge and island. The location of bars may affect the bifurcationthe mechanisms by which channel pattern adjustment and stability and instability. The stable bifurcation may havemaintenance occurs. It showed that local conditions at lower shield stresses than unstable bifurcation.bifurcations affected by upstream supply of sediment andmigration of bars in the upstream channel, making prediction IV. CONTROLLING VARIABLES OF BIFURCATIONof any bifurcation is difficult. The result showed sediment The main controlling parameters are shield stress andsupply upstream of the bifurcation often affected by an aspect ratio of the upstream channel. Larger values of theupstream confluence of two or more merging anabranches. aspect ratio imply a stronger effect of bed from migration on[11] investigated the equilibrium configurations of Y-shaped the system evolution. The bars dynamic will influence thefluvial bifurcation. It concluded that unbalanced equilibrium bifurcation amplitude or bar period increased. This isconfigurations for high values of the aspect ratio and low concluded from numerous studies by previous researches.values of shield stress. Therefore explained those natural From a field study conducted by [3], it was suggested thatbraided rivers concentrate the discharge in a few channels. formation of free bars is controlled by threshold of width to depth ratio of channel depending on shield stress and III. FIELD STUDY ON BIFURCATION roughness. [11] set the dimensionless parameters Field observations highlight various common features characterizing the upstream flow, in determining thedisplayed by various bifurcations. Analysis of multi decadal equilibrium configuration of the bifurcation. The findingstimescales for the spatial and temporal variability of channel showed that if high value of shield stress and low value ofbars was conducted. The spatial and temporal complexity of aspect ratio, the discharge distribution is balanced. Higherbar occurrence demonstrated that more field evidence of the values of the shields parameters of the incoming flow, thedynamics of bars is needed for river management purpose bifurcation kept balanced. Resonant value of aspect ratio, βand for validation of models. Point bars are the dominant type control the direction towards the morphodynamics influence.of bar, as expected both in numbers and area. Study by [12] It controls the planform shape of the channel. Critical valueobserved that mid channel bars started forming close to of aspect ratio sets the occurrence of migrating alternate barsriffles, then related to cross sectional over-widening and after in straight channels.some formative events became attached to one of the banks. [18] highlighted the dimensionless parameters that set byThe changes in sediment supply and in discharge such as water discharge, sediment rate and initial slope that is shieldfrom agriculture, dams or mining have been showed to affect stress, width ratio and sediment size.the formation and number of bars presents in a reach. Thechanges in sediment distribution at a river bifurcations oftenleads to aggradation in one of the downstream that may causeclosure to one of the branches [13] Ds = mean grain diameter; σ = geometric standard deviation [14] carried out series of field campaign in two streams: the of the grain size distribution; υ = shield stress; β = widthSunwapta River and Ridanna Creek. It showed that recurring ratio; the width of the channel at the beginning of eachfeature of observed bifurcations is a strong asymmetry of experiment was set lower than βc. Therefore it may create thetheir morphological characteristics. It is found out that one of free alternated bars which imply stable plane bed; ds =the two distributaries was carrying a far larger discharge. The relative roughnessbranch carrying largest discharge is wider and deeper. The The values of shield stress and width ratio at the onset offormation of an inlet step detected in all monitored bifurcation can be related at least for the runs with uniformbifurcations produce transverse slope that responsible for sediments. The points sits along a critical curve, wherebypartitioning of flow and bed load within the downstream larger values of the shield stress associated with larger valuesdistributaries. of width ratio. The diameter of sediment does not affect this [15] described flow field in Jamuna River, Bangladesh. relationship. Angle of flow, shields stress and channel widthThe results showed flow instability can generate a multi ratio is the main parameters that characterize the flow andthread current and therefore trigger the bifurcation process. channel geometry at the onset of bifurcation.[7] reports two series of field observation from two different [19] had observed that both the transverse bed slope andgravel braided rivers which are characterized by different topographic forcing as potential main controls of thesize and braiding intensity. Bifurcation shows repeated partitioning of flow and bedload at bifurcations. According touneven patterns revealed by unbalance water distribution. It [9] the diffluences are crucial controls on the downstreamis also due to the presence of transverse inlet step that partitioning of flow and sediment. It also central to thedetermine the bed topography at bifurcation and lateral shift different modes of channel changes that have identified inof main flow towards the external bank where erosion braided rivers [20] and control the evolution and sedimentaryconcentrates. architecture of deltaic networks. [16] indicated that stability conditions determine by A field study by [16] stated that flow rate; Q affectssplitting discharge at downstream channel bifurcation. It network upstream of shorelines. A decrease in Q will causeconcluded that for a given shield stress, upstream channel bifurcation to disappear by sediment filled while an increase

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in Q by 60% will create a new channel. During the approach The [24] investigated the consequence of migratingto threshold, increase in Q never cause the abandoned of alternate bars along the upstream channel corresponding tobifurcations whereas increase in shield stress may cause the the transverse division of sediment discharge in the upstreaminstability. channel vary corresponding with time. The result indicated [21] categorized the planimetric configurations of braided that effectiveness of alternating bars in governing thenetworks and concluded that braided stream reach a steady bifurcation essentially depends on ratio between time scale ofstate configuration in terms of belt width, number of branches bars migration and the intrinsic time scale of evolution of theand nodes in a cross section. Two controlling dimensionless bifurcation. If ratio is large, the bar dominates theparameters namely dimensionless discharge and morphological behavior of the system. The migration maydimensionless stream power, scaled with grain size and also induce the complete closure of one of the two branches.intrinsic length scale with two morphological parameters; An extension to the study by [24] was carried out by [21].width and braiding index were introduced. They investigated the roles of migrating bars on water and sediment distribution on a lab-scale Y-shape bifurcation with V. NUMERICAL AND MODELING STUDY OF RIVER fixed and erodible bed composed of well-sorted sand of BIFURCATION which a 1D approach was adopted in their analysis. The results indicate that bifurcation was dominated by bar The river may face varying asymmetrical and unbalances migration where the main flow frequently switches from oneof the system as one of the downstream branch that receive branch to the other until the presence of bar can obviouslylarger discharge is generally deeper and wider. The first close one of the two branches. The main controllingeffort to reproduce these conditions is within the context of parameter is shield stress and aspect ratio of the upstreamone dimensional (1D) approach by [22]. channel that give the stronger effect of bed form migration on The 1D model is recommended to predict the long term the system evolution.evolution of channel based on nodal point condition. The 2D [13] have demonstrated how a bifurcation evolves using aeffects occur close to bifurcation. The simple channel loop 1D and 3D models. The 3D model features marked differencemodel predicts existence of crucial value of shield parameter. in the initial stages due to the formation of local bars andThe 1D approach is to investigate the equilibrium pattern and scour at the bifurcation. This is attributed to the spiralthe stability of gravel bed river bifurcation. Although 1D occurrence at the local bed and flow interaction from threemodel do not allow the detailed description of flow field and axes.bed topography, it is widely used in river engineering for longterm predictions of river morphological development. This VI. CONCLUSIONmodel involves the nodal points where appropriate internalconditions must be imposed. This study set the division of Reviews on the previous studies have provided ansediment discharge within the downstream branches that insight into the mechanics of bar formation that initiatescalled nodal point condition as shown in Fig. 1. bifurcation. Through this review process, possible controlling parameters of bifurcation are recognized. This should facilitate the forthcoming study on the formation of bars that lead to bifurcation.

ACKNOWLEDGMENT The authors express gratitude to the members of Fluvial & River Engineering Dynamics (FRiEnD), Institute of Fig. 1. Scheme of nodal point relationship Infrastructure Engineering & Sustainable Management (IIESM), Faculty of Civil Engineering, andAs a result, the model offered valuable information on the UniversitiTeknologi MARA (UiTM), 40450 Shah Alam,physical mechanism that rules the development of the Malaysia.bifurcation which sediment transport occurred as bedload.Transverse exchange of sediment encourage by topography REFERENCESeffects play significant role to the stability bifurcation and [1] P. Ashmore and D. Smith, “Laboratory Modeling of Gravel Braidedallow equilibrium configuration by different value of flow Stream Morphology,” Water Resources Research, vol. 19, no. 2, Apr.and sediment discharge into the downstream branches. 1983, pp. 392–402. [2] R. Repetto, M. Tubino and C. Paola, “Planimetric instability ofPerformance of bifurcation relate to actual hydraulic channels with variable width,” Journal Fluid Mechanic, vol. 457,conditions and geometry at nodal point. Backwater affects Aug. 2002, pp. 79–109.the equilibrium configuration but not stability because the [3] J. M. Hooke and L. Yorke, “Channel bar dynamics on multi-decadallength of upstream channel is sufficient to prevent changes of timescales in an active meandering river,” Earth Surface Process Landforms, vol. 36, no. 14, Jul. 2011, pp. 1910–1928.boundary conditions at the inlet. [4] G. M. T. Islam, M. R. Kabir, and A. Nishat, “Nodal Point Relation for The extension of study carried out by [23] looked into the the Distribution of Sediments,” Journal of Hydraulic Engineering, no.case of channels with erodible banks that the channel which 10, Oct. 2006, pp. 1105–1109.adjusts their width to the real flow environment. The model [5] M. G. Kleinhans, A. W. E. Wilbers, and W. B. M. Brinke, “Opposite hysteresis of sand and gravel transport upstream and downstream of ashowed that the removal of fixed bank cause one of the two bifurcation during a flood in the River Rhine , the Netherlands,”branches is larger than the other and is fed with greater water Netherlands Journal of Geosciences, vol. 86-3, Aug. 2007, pp.and sediment discharges. The model predicts creation of an 273–285.inlet step that generates a transverse bed slope which [6] R. M. Frings and M. G. Kleinhans, “Complex variations in sediment transport at three large river bifurcations during discharge waves incrucially affects the partition of sediment discharges into the the river Rhine,” Sedimentology, vol. 55, no. 5, pp. 1145–1171, 2008.downstream branches. [7] G. Zolezzi, W. Bertoldi, and M. Tubino, “Morphological analysis and

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D.A. Tholibon, completed her M.Eng in

Environmental Management Engineering, in 2007 from Universiti Teknologi Malaysia, Johor, Malaysia. Currently she is pursuing PhD in Alternative Water Resource in Universiti Teknologi Mara, Selangor, Malaysia. She has published paper in River Hydraulics Engineering. She is a member of MIEM since 2008 until present. Her research interest includes river hydraulic and river engineering.